Glass fibers having non-circular cross sections have hitherto been proposed (for example, see Patent Literature 1). In the present application, a shape in which all the positions on the circumference are separated by an equal distance from the center of the shape is defined as a perfect circle, and shapes different from the perfect circle are defined as non-circular shapes. Examples of the non-circular shapes include a flat shape, a star shape, a cruciform, a polygon and a doughnut shape, and the like. Examples of the flat shape include an elliptical shape, an oval shape and a cocoon shape.
The glass fibers having non-circular cross sections are mixed and melted with a thermoplastic resin, and the resulting molten mixture is injection molded to form a fiber-reinforced resin compact; in this way the glass fibers having non-circular cross sections are regarded to be capable of improving, for example, the mechanical strength, dimensional accuracy and warping of the fiber-reinforced resin compact. The reasons for improving the mechanical strength of the fiber-reinforced resin compact include the fact that the glass fibers having non-circular cross sections have a larger contact area with the thermoplastic resin, as compared to glass fibers having perfectly circular cross sections. The reasons for improving the dimensional accuracy, warping and the like of the fiber-reinforced resin compact include the fact that the glass fibers having non-circular cross sections are more satisfactory in the orientation in the directions along the plane of the compact and tend to be more two-dimensionally oriented at the time of forming the fiber-reinforced resin compact, as compared to glass fibers having perfectly circular cross sections.
The glass fibers having non-circular cross sections are generally formed of E-glass, and the glass fibers composed of E-glass may not be able to attain sufficient strength and sufficient modulus of elasticity. Accordingly, it is desirable to impart sufficient strength and sufficient modulus of elasticity to the glass fibers having non-circular cross sections.
Glass fibers composed of S-glass are known as glass fibers more excellent in strength than the glass fibers composed of E-glass. The glass fibers composed of S glass has a composition in which the content of SiO2 is about 65% by mass, the content of Al2O3 is about 25% by mass and the content of MgO is about 10% by mass, in relation to the total amount of the glass fibers. However, when a glass composition as the raw material for S-glass is melted into molten glass, and glass fibers are obtained by spinning the molten glass, S-glass has a problem that the difference between the 1000-poise temperature of the molten glass and the liquid phase temperature of the molten glass is small.
The “1000-poise temperature” means the temperature at which the viscosity of the molten glass comes to be 1000 poises, and the “liquid phase temperature” means the temperature at which crystals start to precipitate when the temperature of the molten glass is decreased. In general, the appropriate viscosity in the production of glass fibers is said to be 1000 poises or less, and in the foregoing spinning, the wider the temperature range (working temperature range) between the 1000-poise temperature and the liquid phase temperature, the more stably the spinning can be performed; and hence, the working temperature range is used as an indication for ensuring spinnability.
When the difference between the 1000-poise temperature of the molten glass and the liquid phase temperature of the molten glass is small, in the process during which the molten glass is spun and then cooled to be glass fibers, the glass fibers tend to undergo crystallization (devitrification) even under the effect of slight temperature decrease and a problem of breakage of glass fibers or the like tends to occur. S-glass has a narrow working temperature range, and hence, when the glass composition as the raw material for S-glass is melted to prepare a molten glass, it is difficult to stably spin glass fibers having non-circular cross sections from the resulting molten glass. The “devitrification” is the phenomenon that crystals precipitate when the temperature of the molten glass is decreased.
Accordingly, a glass composition has been proposed in which the composition of the glass composition as the raw material for S-glass is improved in such a way that the glass composition includes CaO as well as SiO2, Al2O3 and MgO. As the foregoing glass composition, a glass composition is known which allows the spinning to be easily performed at relatively low temperatures while the working temperature range is being maintained, for example, by decreasing the viscosity on the basis of the decrease of the 1000-poise temperature (see, Patent Literature 2). As the foregoing glass composition, a glass composition is also known in which the difference between the 1000-poise temperature and the liquid phase temperature is large (see, Patent Literature 3).